RFR: 8283237: CallSite should be a sealed class

[liach]

Change `CallSite` to a sealed class, as `CallSite` is an abstract class which does not allow direct subclassing by users per its documentation. Since I don't have a JBS account, I posted the content for the CSR in a GitHub Gist at https://gist.github.com/150d5aa7f8b13a4deddf95969ad39d73 and wish someone can submit a CSR for me.

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Commit messages:
 - 8283237: CallSite should be a sealed class

Changes: https://git.openjdk.java.net/jdk/pull/7840/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7840&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283237
  Stats: 10 lines in 4 files changed: 0 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7840.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7840/head:pull/7840

PR: https://git.openjdk.java.net/jdk/pull/7840